This project is intended for furthering the understanding of scattering mechanisms of ultrasound in biological tissues. This understanding is essentially for improved interpretation of echographic images obtained by ultrasound B-Scanners, as well as for quantitative tissue characterization. Data for blood is needed for the improved design of ultrasonic doppler flowmeters. The specific aims of the project are: (1) to determine the ultrasonic scattering properties of blood at high hematocrits (greater than 35%) under well-defined flow conditions so as to better understand the roles played by the turbulence and erythrocyte aggregation in influencing the scattering strength of blood which has been claimed visualizable with high resolution small-part ultrasonic scanners and (2) to identify the basic mechanisms reponsible for the scattering of ultrasound in tissues by examining the scattering properties of normal and pathological tissues and correlating these results with histological preparations of the tissues. Both the approach used by Shung, Sigelmann and Reid which utilize narrow-band signal and the approach adopted by Bamber, Nicholas and Hill which utilizes broad-band signal will be employed in this investigation. We will carefully compare the two sets of data collected with these two different approaches and inquire into the possible contributing factors should any discrepency exist.